Patent Grant
9633603
Technical Field
The present disclosure relates to the field of organic light emitting display technologies, in particular to a pixel compensating circuit and method of compensating for voltage drop and drift occurring in the threshold voltage of an organic light emitting display device.
Technical Background
An organic light emitting display is a thin film light emitting device that is made of organic semiconductor material and driven by a direct voltage, and includes a very thin organic material coating and a glass substrate. Such organic material of the organic material coating can emit light actively when a current flows there through.
Since a light emitting luminance of the organic light emitting display depends on an amount of the current flowing through the organic light emitting diode, the light emitting luminance, as an electrical property of the driving thin film transistor, will directly affect an display effect of the organic light emitting display, and especially a threshold voltage of the driving thin film transistor often drifts, leading to an uneven luminance problem in the whole organic light emitting display.
In one aspect, an embodiment of the present disclosure discloses a pixel compensating circuit of an organic light emitting display, including: a first transistor, a second transistor, a third transistor, a fourth transistor, a fifth transistor, a driving transistor, a first capacitor and an organic light emitting diode element. The first transistor is controlled by a first driving signal to control transmission of a data signal to a first electrode plate of the first capacitor; the second transistor is controlled by a second driving signal to control transmission of a reference voltage signal to the first electrode plate of the first capacitor; the driving transistor is configured to determine an amount of a driving current which depends on a voltage difference between a gate electrode and a source electrode of the driving transistor; the third transistor is controlled by the first driving signal to control connection and disconnection between the gate electrode and a drain electrode of the driving transistor; the fourth transistor is controlled by a third driving signal to transmit the driving current from the driving transistor to the organic light emitting diode element; the fifth transistor is controlled by a fourth driving signal to control transmission of a supply voltage to the source electrode of the driving transistor; a cathode of the organic light emitting diode element is connected to a low potential, and the organic light emitting diode element is configured to emit light in response to the driving current.
In another aspect, an embodiment of the present disclosure discloses a pixel compensating method of a pixel compensating circuit, where, the first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor and the driving transistor are P-type transistors; or the first transistor, the second transistor, the third transistor, the fourth transistor and the fifth transistor are N-type transistors, but the driving transistor is a P-type transistor; the method includes a node resetting step, a threshold detecting step, a data inputting step and a light emitting step.
In yet another aspect, an embodiment of the present disclosure discloses an organic light emitting display, including: the pixel compensating circuit and organic light emitting diode elements, where, the organic light emitting diode elements emit light in response to the driving current outputted by the pixel compensating circuit.
The present disclosure will be further illustrated in detail below in conjunction with the accompanying drawings and specific embodiments. It may be understood that specific embodiments described herein are merely for explaining the present disclosure rather than limiting the present disclosure. Additionally, it is noted that merely partial contents associated with the present disclosure rather than all contents are illustrated in the accompanying drawings for ease of description.
A first electrode of the first transistor M1 is connected with a data signal line to receive a data signal Vdata, and a second electrode of the first transistor M1 is connected with a second electrode of the second transistor M2 and a first electrode plate of the first capacitor Cst; a first electrode of the second transistor M2 is connected with a reference voltage signal line to receive a reference voltage signal Vref; a source electrode of the driving transistor M0 is connected with a second electrode of the fifth transistor M5, and a drain electrode of the driving transistor M0 is connected with a second electrode of the third transistor M3 and a first electrode of the fourth transistor M4; a first electrode of the third transistor M3 is connected with a gate electrode of the driving transistor M0 and a second electrode plate of the first capacitor Cst; a second electrode of the fourth transistor M4 is connected with the organic light emitting diode element OLED; and a first electrode of the fifth transistor M5 is connected with a supply voltage signal line to receive a supply voltage signal PVDD.
In the pixel compensating circuit of the present embodiment, the first transistor M1 is controlled by a first driving signal S1 to control the transmission of the data signal Vdata to the first electrode plate of the first capacitor Cst; the second transistor M2 is controlled by a second driving signal S2 to control the transmission of the reference voltage signal Vref to the first electrode plate of the first capacitor Cst; the driving transistor M0 is configured to determine an amount of a driving current which depends on a voltage difference between the gate electrode and the source electrode of the driving transistor M0; the third transistor M3 is controlled by the first driving signal S1 to control the connection and disconnection between the gate electrode and the drain electrode of the driving transistor M0; the fourth transistor M4 is controlled by a third driving signal S3 to transmit the driving current from the driving transistor M0 to the organic light emitting diode element OLED; the fifth transistor M5 is controlled by a fourth driving signal S4 to control the transmission of the supply voltage signal PVDD to the source electrode of the driving transistor; and the organic light emitting diode element OLED is configured to emit light in response to the driving current.
Specifically, the first driving signal S1 controls the first transistor M1 and the third transistor M3, the second driving signal S2 controls the second transistor M2, the third driving signal S3 controls the fourth transistor M4, and the fourth driving signal S4 controls the fifth transistor M5, where, Vdata represents the data signal. All of the first driving signal S1, the second driving signal S2, the third driving signal S3 and the fourth driving signal S4 are provided by gate driving lines of the organic light emitting display.
A driving timing of the pixel compensating circuit of the embodiment includes a node resetting stage, a threshold detecting stage, a data inputting stage and a light emitting stage, respectively corresponding to time periods of T11, T12, T13 and T14 in
An operating principle of the pixel compensating circuit of the organic light emitting display according to an embodiment of the present disclosure is illustrated specifically below with reference to
As shown in
As shown in
At this time, a voltage difference Vc between the first electrode plate and the second electrode plate of the first capacitor Cst is calculated by formula (1) below:
Vc=V2−V1=PVDD−Vth−Vdata (1),
where, V2 represents the potential of the second node N2 and V1 represents the potential of the first node N1.
In the threshold detecting stage T12, the voltage difference Vc between the first electrode plate and the second electrode plate of the first capacitor Cst includes the threshold voltage Vth of the driving transistor M0, i.e., the threshold voltage Vth of the driving transistor M0 has been detected in the threshold detecting stage T12, and is stored in the first capacitor Cst.
As shown in
V2′=Vc+V1′=PVDD−Vth−Vdata+Vref (2).
In other words, the data signal Vdata is coupled to the second electrode plate of the first capacitor Cst through the first Capacitor Cst.
As shown in
Vgs=V2′−PVDD=Vref−Vth−Vdata (3).
Since the driving transistor M0 is operated in a saturation region, the driving current flowing through a channel of the driving transistor M0 is determined by the voltage difference between the gate electrode and the source electrode of the driving transistor M0, and the driving current can be obtained based on the electric characteristics of the transistor in the saturation region as follows:
I=K(Vsg−Vth)2=K(Vref−Vdata)2 (4),
where, I represents the driving current generated by the driving transistor M0, K is a constant, Vref represents the reference voltage signal, and Vdata represents the data signal.
Since the fourth transistor M4 is operated in a linear region, the fourth transistor M4 can transmit the driving current I to the organic light emitting diode element OLED, to drive the organic light emitting diode element OLED to emit light for display.
In an implementation of the present embodiment, a signal line of the second driving signal S2 in a pixel can be connected with a signal line of the third driving signal in the preceding pixel, and the signal line of the third driving signal S3 in a pixel can be connected with the signal line of the second driving signal in the next pixel, so that the layout design of an integrated circuit board can further be simplified while implementing the pixel compensating function of the present disclosure.
It is noted that the first transistor M1, the second transistor M2, the third transistor M3, the fourth transistor M4 and the fifth transistor M5 may be N-type transistors but the driving transistor M0 is a P-type transistor in the present embodiment. It can be understood by those skilled in the art that functions of the above-mentioned steps can still be implemented if the first driving signal S1, the second driving signal S2, the third driving signal S3 and the fourth driving signal S4 described above are inversed, which will not be repeatedly described herein.
As can be seen from the above formula (4), the amount of the driving current I is only dependent on the reference voltage signal and the data signal and not dependent on the threshold voltage of the driving transistor and the supply voltage signal, so as to compensate the voltage drop on the power supply line and the threshold voltage, and ensure that in the whole driving process only one of the potentials at both sides of a storage capacitor is changed in order to reduce the impact of a coupling effect of the parasitic capacitor on the node potential, thereby achieving an accurate pixel compensating effect for the organic light emitting display and obtaining a better displaying effect.
As shown in
In the node resetting step 801:
specifically, in this step, both of the first driving signal and the third driving signal are at a low level, and both of the second driving signal and the fourth driving signal are at a high level, so that the first transistor, the third transistor, the fourth transistor and the driving transistor are turned on, and the second transistor and the fifth transistor are turned off. The data signal is transmitted to the first electrode plate of the first capacitor through the first transistor. The gate electrode of the driving transistor and the second electrode plate of the first capacitor Csttake on a low potential of the cathode of the organic light emitting diode element.
In the threshold detecting step 802:
specifically, in this step, the first driving signal is at a low level, the second driving signal is at a high level, the third driving signal changes from a low level to a high level, and the fourth driving signal changes from a high level to a low level, so that the first transistor, the third transistor and the fifth transistor are turned on, the second transistor and the fourth transistor are turned off, and the driving transistor is turned off when the voltage difference between the gate electrode and the source electrode of the driving transistor is equal to a threshold voltage of the driving transistor. When the driving transistor is turned off, the threshold voltage of the driving transistor is stored in the first capacitor.
In the data inputting step 803:
specifically, in this step, the first driving signal changes from a low level to a high level, the second driving signal changes from a high level to a low level, and the third driving signal is at a high level, so that the first transistor, the third transistor, the fourth transistor and the driving transistor are turned off, and the second transistor is turned on. The data signal is coupled to the second electrode plate of the first capacitor through the first capacitor. The reference voltage signal is transmitted to the first electrode plate of the first capacitor.
In the light emitting step 804:
specifically, in this step, the first driving signal is at a high level, the second driving signal is at a low level, the third driving signal changes from a high level to a low level, and the fourth driving signal is at a low level, so that the first transistor and the third transistor are turned off, and the second transistor, the fourth transistor and the fifth transistor are turned on, and the driving current of the driving transistor is determined by the voltage difference between the gate electrode and the source electrode of the driving transistor. The fourth transistor transmits the driving current to the organic light emitting diode element, and the organic light emitting diode element emits light in response to the driving current.
Further, in the node resetting step (i.e. within the timing T21), the fourth driving signal changes from a low level to a high level before the first driving signal changes from a high level to a low level; and the fourth driving signal changes again from a high level to a low level after the third driving signal changes from a low level to a high level. Since the nodes N1 and N2 are reset in the node resetting step (i.e. within the timing T21) only if both of the first driving signal S1 and the third driving signal S3 are at a low level and all of the first transistor M1, the third transistor M3 and the fourth transistor M4 are turned on, the reduction of the current flowing through the light emitting diode element OLED can be ensured as long as the fourth driving signal S4 is at a high level to turn off the fifth transistor in this step, thus decreasing the luminance under a dark state and improving a contrast of the organic light emitting display product.
In the present embodiment, the timings of the second driving signal S2 and the third driving signal S3 and of each signal in the data inputting step (i.e. within the timing T23) and the light emitting step (i.e. within the timing T24) are the same as those as previously described, and thus will not be repeated herein for the sake of brevity.
It is noted that in the present embodiment, the first transistor M1, the second transistor M2, the third transistor M3, the fourth transistor M4 and the fifth transistor M5 may also be N-type transistors but the driving transistor M0 may be a P-type transistor. It can be understood by those skilled in the art that functions of the above-mentioned steps can still be implemented as long as the first driving signal S1, the second driving signal S2, the third driving signal S4 and the fourth driving signal S4 described above are inverted. That is, when the first transistor, the second transistor, the third transistor and the fourth transistor are N-type transistors and the driving transistor is a P-type transistor.
In the node resetting step, both of the first driving signal and the third driving signal are at a high level and both of the second driving signal and the fourth driving signal are at a low level, so that all of the first transistor, the third transistor, the fourth transistor and the driving transistor are turned on, and the second transistor and the fifth transistor are turned off.
In the threshold detecting step, the first driving signal is at a high level, the second driving signal is at a low level, the third driving signal changes from a high level to a low level, and the fourth driving signal changes from a low level to a high level, so that all of the first transistor, the third transistor and the fifth transistor are turned on, the second transistor and the fourth transistor are turned off, and the driving transistor are turned off when the voltage difference between the gate electrode and the source electrode of the driving transistor is equal to a threshold voltage thereof.
In the data inputting step, the first driving signal changes from a high level to a low level, the second driving signal changes from a low level to a high level, and the third driving signal is at a low level, so that the first transistor, the third transistor, the fourth transistor and the driving transistor are turned off, and the second transistor is turned on.
In the light emitting step, the first driving signal is at a low level, the second driving signal is at a high level, the third driving signal changes from a low level to a high level, and the fourth driving signal is at a high level, so that the first transistor and the third transistor are turned off, and the second transistor, the fourth transistor and the fifth transistor are turned on, and the driving current of the driving transistor is determined by the voltage difference between the gate electrode and the source electrode of the driving transistor.
In the embodiment, the voltage drop on the power supply line and the threshold voltage drift are compensated and it is ensured that in the whole driving process only one of the potentials at both sides of the storage capacitor is changed in order to reduce the impact of a coupling effect of a parasitic capacitor on the node potential, thereby obtaining a better displaying effect.
It is noted that the preferred embodiments and the technology principles of the present disclosure are merely described as above. It will be understood by those skilled in the art that the disclosure is not limited to particular embodiments described herein. Various changes, readjustment and substitutions can be made to the present disclosure by those skilled in the art without departing from the scope of protection of the present disclosure. Therefore, although the disclosure is illustrated in detail through the above embodiments, it is not merely limited to the above embodiments, and can further include more others equivalent embodiments without departing from the conception of the present disclosure. The scope of the disclosure is determined by the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014 1 0245542 | Jun 2014 | CN | national |
This application is a Continuation of U.S. application Ser. No. 14/479,572, which application claims priority to Chinese Application No. 201410245542.5, filed on Jun. 4, 2014, the contents of which are incorporated herein by reference in their entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 7057588 | Asano | Jun 2006 | B2 |
| 20050057459 | Miyazawa | Mar 2005 | A1 |
| 20050269959 | Uchino | Dec 2005 | A1 |
| 20060103322 | Chung | May 2006 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20160307508 A1 | Oct 2016 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14479572 | Sep 2014 | US |
| Child | 15194540 | US |
